Excel is expecting the data in the dd/mm/yyyy format and some of the
dates that are mm/dd/yyyy would appear to meet this format so they have
become a date (all be it incorrect), the others it has just recognised
as a text string

If the value is in the cell A1 there are 2 things you need to be able
to do
1) if the cell is a date it needs to have the month and date switched
2) if the date is text then it needs to be converted into a date

As dates are number the following should work although you will need to
format the cell yyyy-mm-dd

in another cell say b1 put
=IF(ISNUMBER(A1),DATE(YEAR(A1),DAY(A1),MONTH(A1)), DATE(RIGHT(A1,4),LEFT(A1,2),MID(A1,4,2)))

copy this down by the side of all your dates

then copy all of it and paste special on top of itsself as values to
loose the formulas

copy again and paste on top of the original data, you can now delete
the column you added

If this does not work please get back to me

Regards

Dav


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